The prevention of the formation of pressure sores is a desirable goal when designing a wheelchair cushion. It is known that skin that is subjected to high pressure, high temperatures, moisture build up, and shear over prolonged periods of time is susceptible to tissue stress and breakdown that can cause the formation of pressure sores. While heat and moisture are generally believed to be significant factors that lead to the formation of pressure sores, the relative contribution of these factors as causative agents for pressure sores is only now beginning to be understood. There is now evidence that the presence of constant pressure on tissue combined with increasing temperature generates a proportional increase in pressure sore formation. It has been shown that increases in the moisture level generally follow rises in skin temperature, and that moisture is also a significant influence in the formation of pressure sores. Therefore, reducing skin temperature also correlates to a reduction of moisture accumulation in the seating environment, further reducing the likelihood of pressure sore formation.
The need to reduce the maximum temperature and the time averaged temperatures to which the skin is exposed is demonstrated by the fact that metabolic tissue stress increases with rising temperature. This concept is predicted by the Arrhenius equation:k=A*exp(−Ea/R*T)where k is the rate coefficient, A is a constant, Ea is the activation energy, R is the universal gas constant, and T is the temperature (in degrees Kelvin). R has the value of 8.314×10−3 kJ mol−1K−1.
The equation indicates that the biochemical reaction rate, and thus the cellular metabolic rate, increases as temperature increases. Application of the Arrhenius equation shows that for every 10° C. increase in temperature, the reaction rate and thus metabolic substrate requirement doubles. Correspondingly, as skin temperature goes from 28° C. to 35° C., which represents the rise of normal skin temperature in air to temperature when seated, the tissue metabolic rate would show a 50% increase. This metabolic rate increase occurs at the same time that the load created by sitting is applied to the tissue of a buttock, restricting or occluding blood flow by mechanical compression. As a result, the buttock tissue has increased demand for oxygen and metabolic nutrients, while at the same time diminishing their availability, potentially resulting in cellular metabolic deficit or cellular stress. In addition, the construction materials used in seat cushions, as well as clothing materials worn by the user or blankets, wraps, or the like, upon which the user might be seated, are typically found to be insulative, making the temperature related stress on skin under these conditions even worse. The insulative nature of these layers can, for example, maintain up to a 13° C. temperature difference between core temperature and outer layer surface temperature. Thus, a cushion that could reduce the temperature on the user's skin under these conditions would be desirable.
Stability of the wheel chair cushion is another important factor. Stability is normally defined as the ability of a cushion to maintain the cushion user in the same position as when they were seated or, at least to reduce movement of the user from the cushion. Many currently available cushions allow the user to slide, shift, or to rock from side to side during use of the wheelchair. Typically, providing stability in a proximal area, that is in the pelvic area, will improve stability throughout the lower portion of the user's body. Therefore, a cushion designed to improve the stability of the user within the wheelchair would be desirable.